Mutations in the Arabidopsis pauses (psd) gene result in the death of undifferentiated cells in the shoot apical meristem, a delay in the production of leaf primordial, and heterochronix shifts in leaf identity. PSD encodes a homologue of exportin-t, a gene required for the nuclear export of mature tRNA molecules. As very few members of the importin/ exportin family have been characterized in multi cellular organisms, several approaches will be taken to understand the role of PSD in Arabidosis development. Specifically: 1) the spatial and temporal requirements for PSD expression will be determined by characterization of the mRNA and protein expression patterns and by targeted expression of the PSD gene, 2) genetic and molecular analysis will be used to assay tRNA export and to determine the effect of the PSD mutation on genes involved in meristern development, leaf initiate, and phase change, and 3) potential downstream factors will be isolated in a screen for enhances and suppressors of the PSD phenotype. These experiments will offer insight into the maintenance and differentiation of pluripotent cells and the regulation of developmental 'clocks.' They will also provide a rare opportunity for the in vivo analysis of nuclear transport machinery during the development of a multi cellular organism.